If you are already used to playing with a click like that, then a pretty simple solution would be a small mixer like this one. You can put the monitor signal from the board into one channel and the click into the other.

FYI, I suggested that board because it has a built-in compressor on the first few channels. This acts as a limiter so feedback or sudden "Are you ready to rock, Springfield!!" lead singer screams won't blow out your ears.

Another option is the Tasty Blender. This is one that is more customized for guitarists and drummers. I have the first-gen model that is just a single input, but this one has multiple inputs. It also has a limiter.

For the actual in-ear monitors, I would recommend a set of the Shure SE215 or equivalent.

Pros:
In-ears really help with making the monitor mix more consistent and once you get used to hearing everything at a nice comfortably lower volume you won't miss the "amp headache" the next day after playing.

Cons:
You can get a "closed" off feeling from the rest of the band or the audience. Getting a good monitor mix is key because you can't just "lean over to the right in order to hear a little more guitar during the solo" and you can't adjust your dynamics off of the rest of the band.

Overall, affordable and good quality in-ears are a good investment and will help you in the long run. I wish that I had them 20 years ago.